1. Field of the Invention
The invention relates to a vibration damping system, especially for use in the damping of vibrations, e.g., from trains, other traffic and damping of ground borne vibrations in general.
2. The Prior Art
In the prior art, it is well known to incorporate elastic material under traffic lines and in particular under tracks for trains, trolley busses, tramcars and similar traffic lines in order to damp the vibrations caused by this heavy traffic. In the prior art material layers of elastic material especially made from rubber, PUR-foams and cork, respectively, as well as combinations thereof, have been used for damping such vibrations.
One of the preferred materials for the damping of vibrations has so far been plates or mats of vulcanised rubber which have excellent elastic properties for use as vibration damping material. Vibration damping constructions, wherein the vibration damping elements are constituted by rubber, have in most situations an acceptable vibration damping efficiency; however, the amount of rubber necessary in such constructions in many situations results in a relatively expensive product. Furthermore, there can be a general interest to avoid or reduce the use of rubber materials due to environmental pollution during its production and pollution due to escape of additives, e.g., softening additives during use in moist environments, U.S. Patent No. 5,060,856 describes such an elastomeric mat for use, e.g., in damping of the sound from trains.
It has also been tried to use a mineral fibre board as sound damping material in railway construction, e.g. as disclosed in DE 35 27 829 and in EP patent publication no. 922 808. This sound damping system has shown to be very good in certain situations.
In general, it has been found that the use of mineral fibre mats or boards in vibration damping systems for railway foundations is highly desirable due to adequate performance, easy installation, 100% recycling ability, low pollution effect and a competitive price. However with the known mineral fibreboards there is a risk, when it is used over a long period under high loads, such as the forces from ballast gravels during passage of train, that this may have an effect on the mineral fibre material over time. This ageing effect is also seen with some of the known rubber and PUR-materials.
By incorporating the above materials in railway tracks for damping vibration, it has been observed that there is a risk that the load from passing trains imposed in the vibration damping system causes an ageing of such system over time. Such ageing is characterized by a change in static and dynamic stiffness of the anti-vibration plate of the vibration damping system, which is unwanted. For instance, the static and the dynamic stiffness of the anti-vibration plate may decrease significantly during the first 5 to 10 years of use.
Normally it is desired that a vibration damping system under railways should have a durability of about 40 years. A minimum demand from Deutsche Bahn-Norm (Technische Lieferbedingungen BN 918 071-1, September 2000) is that through mechanical excitation the static stiffness of the anti-vibration plate of the vibration damping system may not decrease or increase more than about 10-20% during a simulated 40 years period in the laboratory.
According to standard and practical experience the static and dynamic stiffness should preferably be substantially constant over time.
Accordingly, there remains a need for a vibration damping system of the above-mentioned kind which does not exhibit the above-identified drawbacks.
The object of the invention is therefore to provide a vibration damping system comprising an anti-vibration plate with improved stability with respect to static and particularly dynamic stiffness, and preferably comprising an anti-vibration plate with a substantially constant static and dynamic stiffness during its life time defined as 40 years.
Another object of the invention is to provide a vibration damping system comprising an anti-vibration plate having an upper surface which is sufficiently strong to withstand and distribute the forces from the ballast layer. This surface protects the bottom layer or the elastic part of the mineral fibre board. Furthermore, the surface should be designed so that the ballast layer can be replaced. The ballast layer replacement may be carried out three or four times within the lifetime of the vibration damping system.